This invention relates to the electrochemical separation of chlorine gas from a mixture with other gases. In particular, it relates to the electrochemical separation of chlorine gas from the tail gas streams of chlor-alkali plants.
In a chlor-alkali plant, chlorine is generated by electrolysis of an aqueous sodium chloride solution. The chlorine is saturated with brine water vapor at .about.95.degree. C. at pressures ranging from slightly negative or positive to, in some membrane cell plants, up to several hundred kilopascals. In the diaphragm process, the chlorine gas stream also carries along droplets of sodium sulfate and salt.
After drying the chlorine using concentrated (96-98 wt %) sulfuric acid, the chlorine gas is passed through demisters to remove sulfuric acid mist. To avoid the formation of chlorine hydrate crystals, the chlorine is first cooled to usually not less than 10.degree. C. Cooling is carried out indirectly in tubular titanium heat exchangers in one or two stages using chilled water on the coolest stage. Water and remaining solids are removed in eitherwet Brinks demisters, which have special filter elements containing glass wool fibers, or in electrostatic precipitators. The cooled, dry chlorine gas is scrubbed with liquid chlorine to pre-cool it prior to compression. About half of the chlorine produced is used as cooled dry gas, which is transported by pipelines to the consuming process; the remainder is liquefied, stored, and shipped.
The tail gas (also called as vent or sniff gas) from the liquefaction process contains gases such as Cl.sub.2, O.sub.2, N.sub.2, CO.sub.2, and H.sub.2 --the exact composition being dependent on the cell current efficiency and other process variables. It is essential that the Cl.sub.2 in the tail gas be recovered, as it cannot be vented due to air quality regulations.
There are several technologies available to recover the Cl.sub.2 from this tail gas. Some of these technologies involve selectively absorbing the chlorine in water, carbon tetrachloride, or onto solid absorbents. The chlorine can also be reacted with sulfur to produce sulfur monochloride, hydrogen to produce HCl, or water to produce chlorine hydrate. Permselective membranes can separate the chlorine from the other gases. However, these technologies all have problems, ranging from the use of an ozone depleter (CCl.sub.4), to high capital costs, corrosion of equipment, poor efficiency, and inability to remove CO.sub.2 from the chlorine. At present, no viable, energy efficient technology is available to recover chlorine, and new plants are often designed to neutralize chlorine from the tail gas with caustic and dispose off the effluent.